Medical monitors often include recording devices for memorializing on a paper medium information detected by the monitor. Typically, the paper medium is a chart or graph with the information plotted on the paper medium by a printer of the recording device in the form of a curve. It is therefore critical in accurately plotting the curve that the paper medium be properly positioned relative to the printer.
For example, in recording fetal heart rates, guard rails which are located on either side of a paper roller serve as stops to prevent the lateral (i.e. side to side) movememt of the strip chart paper as the printer plots the fetal heart rate on the paper. These guard rails are distanced from each other by a length sufficient to allow for a maximum variation in the width of the paper due to manufacturing tolerances. Consequently, strip chart paper having a width which is less than the fixed distance between the guard rails is able to travel therebetween. This can be expecially troublesome since the fetal heart rate scale extends in the same diretion as the width of the strip chart paper. In particular, since the distance between incremental fetal heart rates (referred to as beats) is only approximately 2/1000 of an inch, even slight deviations from the desired path of the paper due to the paper drifting between the guard rails will result in heart rates incorrectly recorded.
Furthermore, quite often when the printer stops plotting, whether for a moment or longer, the path of the strip chart paper changes direction within the boundaries set by the guard rails. Therefore, in order to reduce the tendency of the paper to deviate from its desired path the paper needs to be repositioned relative to the printer and advanced a number of pages beyond the printer prior to turning the printer on again.